CLASSIFICATION OF ROCKS AND PROPERTIES OF MINERALS

CLASSIFICATION OF ROCKS AND PROPERTIES OF MINERALS

1. Minerals II: Physical Properties and Crystal Forms
From: http://webmineral.com/data/Rhodochrosite.shtml

2. Physical properties of minerals
•In a previous lecture, we discussed that
minerals have both a definite chemical
formula and a definite crystal lattice –
both of which determine the properties
of the mineral
•In this lecture, we will discuss what
those properties are!

3. The Physical Properties of Minerals
•Color
•Streak
•Luster
•Hardness
•Cleavage/Fracture
•Crystal Form
•Specific Gravity
•Special Properties

4. Important Physical Properties II
•Color - Although an
obvious feature, it is
often unreliable to use
to determine the type
of mineral.
–Color of a mineral may be
useful for determining any
impurities that are present
in the mineral
–For example – Rose quartz
has iron impurities; green
quartz has chlorite
impurities

5. Important Physical Properties III
•Streak - The color of a mineral in its
powdered form; obtained by rubbing the
mineral against an unglazed porcelain plate.

–Streak is usually less variable than color.
–Streak can be VERY different from color!

6. Important Physical Properties I
•Luster - This property describes
the appearance of reflected light
from the mineral's surface.
•Luster is either “metallic” or
“nonmetallic”
•Nonmetallic minerals are described
using the following terms: vitreous,
pearly, silky, resinous, and earthy.

7. Important Physical Properties V
•Hardness - This is the resistance of the mineral
to abrasion or scratching. This property doesn't
vary greatly from sample to sample of the same
mineral, and thus is highly diagnostic.
• A value is obtained by comparing the mineral to
a standard scale devised by Moh, which is
comprised of 10 minerals ranging in hardness
from talc (softest) to diamond (hardest).

8. Fingernail Hardness (2.5) Scratches Gypsum (2)

9. Mohs’ Hardness Scale

10. Important Physical Properties VI
•Cleavage - Orientation and number of planes of
weakness within a mineral. Directly reflects the
orientation of weak bonds within the crystal
structure. This feature is also very useful
•Fracture - This describes how a mineral breaks
if it is not along well defined planes. In minerals
with low symmetry and highly interconnected
atomic networks, irregular fracture is common.

11. Planer Cleavage in Mica

12. Weak Bonding Yields Basal Cleavage

13. Rhombohedral Cleavage in Calcite

14. Conchoidal Fracture in Glass

15. Important Physical Properties IV
•Crystal form or habit - The shape of crystals
generally reflects the internal arrangement of their atoms
in their crystal lattice.
•This can be altered, however, if the mineral crystallized in
an environment that did not allow it to grow properly.
•Imagine wearing shoes that are too tight – your foot will
change its shape to match the shoes!

16. Crystal Forms: Quartz

17. Feldspar

18. Intergrown cubic crystals of fluorite

19. Density and Specific Gravity
•Density - Defined as the mass divided by
the volume.
•Specific Gravity - Ratio of the mass of a
substance to the mass of an equal volume of
water. Note that D
water = 1 g/cm
3
. S.G. is
unitless.
•
Examples - quartz (SiO
2) has a S.G. of 2.65
while galena (PbS) has a S.G. of 7.5 and
gold (Au) has a S.G. of 19.3.

20. Special Properties
•
Magnetism - Magnetite (Fe
3
O
4
) has this property
and it can be used to distinguish it from other
non-magnetite iron oxides, such as hematite
(Fe
2O
3).
•Double Refraction - Seen in calcite crystals.
Light is split or refracted into two components
giving rise to two distinct images.
•Taste/Smell – Halite has a salty taste; Sulfur
smells of rotten eggs
•Reactions with acid – Calcite fizzes on reaction
with acid

21. Plagioclase
striations

22. Calcite Double Refraction